Systems for treating waste water, such as sewage and/or industrial waste, often include primary and secondary treatment means, the primary means usually comprising a treatment step which is aimed at separating solid materials and removing the great bulk thereof from the waste waters. This step is often followed by secondary treatment means in which the waste water is further treated to achieve a substantially clarified effluent which resembles clear water, but which is still not sufficiently treated that it can be lawfully discharged into rivers and streams, or onto the surface of the earth. Although the various jurisdictions have different requirements with respect to sufficiency of treatment of the waste water before it can be discharged, most of them require some further treatment of the effluent beyond the above mentioned secondary clarification step. The primary and secondary treatment means, hereinafter referred to as the main treatment means, can employ activated bacteria in the presence of a degree of aeration sufficient to support the bacteria. This sort of main treatment can be achieved using systems of the type disclosed for instance in the copending patent application of Peasley and McKinney, Ser. No. 579,176, filed May 20, 1975, now U.S. Pat. No. 3,984,322, and entitled "SEWAGE TREATMENT APPARATUS", and as disclosed in the copending application of Peasley, Serial No. 641,867, filed Dec. 17, 1975, and entitled "APPARATUS FOR TREATING SEWAGE". Systems for the further treatment of clarified waste water before it can be dumped are frequently referred to as tertiary systems, and it is to improvements in this type of system that the present invention relates.